Compared to mosquitoes, gnats, stable flies, deerflies and other insects that bedevil horses, botflies seem relatively benign. After all, they don't bite--adult botflies do not even have fully developed mouth parts--and their main preoccupation in life is mating and laying eggs.
Yet, as any horsekeeper can attest, botflies are persistent and troublesome winged pests. Persistent, because to procreate they must reach their target species. Unlike biting flies, which feed on many species and can be deflected to other victims, nothing short of death will stop a female equine botfly from getting to a horse to lay her eggs. And troublesome, because the larvae that hatch from those eggs head toward the horse's stomach or small intestine, where they implant themselves and spend most of their life span as internal parasites.
Botfly larvae are not among the most destructive of the internal parasites--nematodes such as strongyles and roundworms hold that distinction. But large populations of bot larvae in the gut have been implicated in mild colics, occasional blockages and--in rare cases--fatal perforations of the stomach. Fortunately, you can keep bots under control with judicious use of dewormers, along with a few preventive measures. Here's what you need to know to protect your horse from botflies.
A Fly's Life
Adult botflies are distinctive in appearance and behavior. "They resemble a small bumblebee--brown and hairy," says Craig Reinemeyer, DVM, PhD, a parasitologist with East Tennessee Clinical Research. The females hover around horses in a somewhat C-shaped posture, ready to lay eggs as soon as they dart onto the target site.
Horses have no difficulty recognizing approaching botflies. "The horse is irritated or sensitized to the vibration or buzzing of the fly and reacts strongly," says Dennis French, DVM, professor of farm animal health management at Louisiana State University. To dodge botflies, horses may run, seek protection in thick brush or stand in deep water. To protect their heads they may stand together and rub their chins on each other's backs.
Three species of equine botflies are found in the United States, and you can tell them apart by observing where on the horse's body they prefer to lay their eggs. By far the most common equine botfly is Gasterophilus intestinalis. These are the flies that lay pale yellow eggs on a horse's legs as well as on the mane, shoulders and flanks. Each female can lay 150 to 500 eggs, one to a hair.
Another common botfly is G. nasalis, which lays its yellow eggs under the chin or lower jaw. These eggs are harder to see since they are deposited between the hairs, and you often have to part the hair with your fingers to find them.
"In the United States probably 95 percent of horses would have G. intestinalis if you didn't treat them to get rid of bots. This species is ubiquitous," says Reinemeyer. "By contrast, only about 30 to 50 percent of horses will have G. nasalis." In fact, for a recent study he conducted, Reinemeyer needed to find horses infected with G. nasalis. "We needed 17 horses for the study, and we had to scope more than 50 horses to find enough," he says.
A third type of bot, G. haemorrhoidalis, is very rare in the United States. This species lays clusters of black eggs on the muzzle near the horse's lips.
The egg-laying habits of botfly species may differ, but once hatched their larvae all have the same goal: to get into the horse's mouth. Those whose eggs were laid on the head move there on their own; those on the legs and other parts of the body need to attach themselves to the horse's lips or tongue whenever the muzzle is in the vicinity. It might sound like a tricky bit of timing, but in fact the G. intestinalis larvae are well adapted to perform this feat: As the horse licks or rubs his legs, the heat, moisture and carbon dioxide in his breath stimulate the eggs to hatch very quickly.
Researchers have witnessed this phenomenon using a microscope. "If you breathe on them, they immediately hatch," says Jack Campbell, PhD, veterinary entomologist at University of Nebraska's West Central Research and Extension Center. "I've had them in a petri dish, getting ready to photograph them, and if you happen to breathe on them, you can see the egg opening up as they come out the end of it."